More precisely, the invention concerns an exhaust diffuser for a gas turbine, the diffuser being designed to be inserted downstream from a last expansion stage constituted by an outlet duct of circular cross-section containing a central bulb.
The gases from gas turbines leave the last expansion stage at very high speeds. It is necessary to reduce such speeds considerably before discharging the gases into the atmosphere, so as to ensure that the exhaust equipment remains reliable by reducing the stresses due to the flow, so as to enhance the performance levels of the turbine by limiting the head loss of the flow, and so as to be capable of providing appropriate treatment for the noise emitted by the exhaust of the turbine.
It is therefore essential to master the problem of diffusing the flow of the exhaust gases if the overall performance levels of a gas turbine are to be mastered technically.
The dimensions of the ducts and of the diffusing members also have a non-negligible economic impact, especially when the sizes of turbines are tending to increase, as is now the case, and the compactness of the exhaust ducts is another essential parameter in making a high-performance gas exhaust system.
Gas turbine exhaust gases are exhausted via a duct of circular cross-section, and the stream of air discharged thereby is to a certain extent annular in cross-section given the central bulb. The end of the central bulb and therefore the annular flow cross-section is the most critical region of the diffusion. The limits and the sources of instability of the diffusion are to be found in that region.
Generally speaking, the outlet of the duct of circular cross-section must be connected to a duct of square cross-section so as to obtain a duct that can be used for installing silencer devices, and also to obtain overall compactness of the ducts. This transition limits the possibility of diffusing the gases correctly, in stable manner and with homogeneous speed profiles.